Mechanochemical oxidation method

ABSTRACT

The invention provides a method for preparing an actinide metal peroxide from a corresponding actinide metal oxide under solid reaction conditions.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application Number 63/292,248 that was filed on Dec. 21, 2021. The entire content of the application referenced above is hereby incorporated by reference herein.

BACKGROUND

Formation of hexavalent uranium U(VI) peroxide complexes and solids has been demonstrated in chemically diverse environments and is particularly important to the fuel cycle for light-water nuclear reactors. U(VI) peroxides have been identified in natural geologic systems as mineral phases (studtite and metastudtite) within ore deposites (K.-A. H. Kubatko, et al., Science, 2003, 302, 1191-1193). Studtite and metastudtite are the only known peroxide bearing minerals and their presence as naturally occurring phases demonstrates the stability of the peroxide within U(VI) solids (K.-A. H. Kubatko, et al., Science, 2003, 302, 1191-1193). These U(VI) peroxide phases are also industrially relevant materials for the front-end of the nuclear fuel cycle (South Africa Pat., ZA patent document 76/2451/A/, 1976; and R. C. Ewing, Corrosion of spent nuclear fuel: the long-term assessment, University of Michigan, Ann Arbor, Mich. (US), 2003). In the mining process, peroxide is introduced to the system during an in-situ acid leaching of ore to oxidize the uranium from the tetravalent oxidation state to the more soluble hexavalent form (C. R. Edwards and A. J. Oliver, JOM, 2000, 52, 12-20). Later in the process, U(VI) peroxide becomes a major constituent of commercial yellowcake due to the use of aqueous H₂O₂ during the precipitation step (Caropreso, F. E.; Kreuz, D. F. Production of uranium peroxide. CA patent document 1076365/A/, 1980.; and M. A. E. Abdel-Rahman and S. A. El-Mongy, Zeitschrift für anorganische and allgemeine Chemie, 2018, 644, 29-32). In addition, the high radiation fields associated with used nuclear fuel induces the formation of these U(VI) peroxide phases, making them an important consideration for stability of the waste forms (X. Guo, S. V. et al., Proceedings of the National Academy of Sciences, 2014, 111, 17737). Studtite and metastudtite have been observed as corrosion products on surfaces of used nuclear fuel within laboratory experiments (A. Canizares, et al., Journal of Raman Spectroscopy, 2012, 43, 1492-1497), nuclear fuel suspended in cooling ponds (D. H. Brady, et al., Radiochimica Acta, 2005, 93, 159-168), and as alteration phases of corium at the Chernobyl site (B. Zubekhina, B. et al., Sustainability, 2021, 13, 1073; B. Zubekhina, B. et al., Sustainability, 2021, 13, 1073; and W. J. Gray and I. R. Triay, Scientific Basis for Nuclear Waste Management XX, Materials Research Society, 1997).

Given the importance of U(VI) peroxide phases within the nuclear fuel cycle, developing a robust understanding of the formation of such materials can lead to advances in fuel fabrication and waste reprocessing. Among the literature reports for U(VI) peroxide synthetic protocols, there are three major synthetic pathways leading towards their formation. First and most common is the direct addition of 30% aqueous H₂O₂ (L. Silverman, R. Sallach, R. Seitz and W. Bradshaw, Industrial and Engineering Chemistry, 1958, 50, 1785-1786). This straightforward approach is utilized in industrial processes on the front-end of the fuel cycle and is used in the laboratory setting to evaluate the basic chemistry of U(VI) peroxides (T. Sato, Journal of Applied Chemistry, 1963, 13, 361-365). Direct addition of hydrogen peroxide over the range of pH values, within mixed solvents systems, and in presence of varying counter ions and chelating organic ligands has resulted in the isolation of numerous uranyl peroxide nanoclusters, displaying vast structural diversity of uranyl peroxides in both solution and solid-state (P. C. Burns, Mineralogical Magazine, 2011, 75, 1-25; P. C. Burns and M. Nyman, Dalton Transactions, 2018, 47, 5916-5927; and A. Arteaga, et al., Chemistry—A European Journal, 2019, 25, 6087-6091). Second, U(VI) peroxide phases can be formed by a photochemical reaction with UV light (W. L. Waltz, et al., Inorganica Chimica Acta, 1999, 285, 322-325). This mechanism relies on photoexcitation of the uranyl (U(VI)O₂ ²⁺) cation, allowing the U≡O_(y1) to perform H-abstraction on the organic substrate, leaving behind a C-centred radical (S. G. Thangavelu and C. L. Cahill, Inorganic Chemistry, 2015, 54, 4208-4221). The organic radical then scavenges dissolved molecular O₂ from the solution, resulting in reactive O₂ species that can be intercepted by the uranyl cation (A. S. Jayasinghe, et al., Crystal Growth & Design, 2019, 19, 1756-1766; and D. V. Kravchuk and T. Z. Forbes, Angewandte Chemie International Edition, 2019, 58, 18429-18433). Last, radiolysis of H₂O molecules under aerated conditions leads to radical cascade reactions that can result in the formation of U(VI) peroxides (J. F. Lucchini, C. Jegou, G. Sattonnay, C. Ardois and C. Corbel, France, International Conference Proceeding, Atalanta, 2000, 1-7). The major products of these radical cascades include superoxide radicals O₂ ⁻·, hydroperoxyl radicals OOH·, hydroperoxyl anions OOH⁻, peroxide anions O₂ ²⁻, and molecular hydrogen peroxides H₂O₂ (R. Radi, Proceedings of the National Academy of Sciences, 2018, 115, 5839; M. Hayyan, et al., Chemical Reviews, 2016, 116, 3029-3085; P. Alexander and D. Rosen, Nature, 1960, 188, 574-575; and D. V. Kravchuk, et al., Angewandte Chemie International Edition, 2021, 60, 15041-15048). All of these species can potentially complex with U(VI) to form peroxide phases and this is the synthetic pathway that has been suggested for the observation of studtite and metastudtite on the surface of used fuel (J. F. Lucchini, C. Jegou, G. Sattonnay, C. Ardois and C. Corbel, France, International Conference Proceeding, Atalanta, 2000, 1-7; L. Sarrasin, et al., The Journal of Physical Chemistry C, 2021, 125, 19209-19218; and A. J. Popel, et al., Journal of Alloys and Compounds, 2018, 735, 1350-1356).

While current synthetic methods have led to advancements in our understanding of U(VI) peroxide chemistry, the major challenges include difficulties in controlling the chemical environment and formation of radioactive and corrosive waste streams (R. C. Ewing, Nature Materials, 2015, 14, 252-257; and P. C. Burns, R. C. Ewing and A. Navrotsky, Science, 2012, 335, 1184). The use of 30% (9.8 M) H₂O₂ results in a very reactive chemical environment that can induce breakdown of the peroxide under basic conditions (F. R. Duke and T. W. Haas, The Journal of Physical Chemistry, 1961, 65, 304-306). This leads to reports of fizzing and bubbling of the reaction and variability in the total concentration of peroxide present in the system (S. Hickam, et al., Inorganic Chemistry, 2019, 58, 5858-5864). While this has resulted in a larger number of U(VI) peroxide phases and topologies, it is difficult to precisely control the reaction conditions.

Photochemical routes typically lead to low yields and limited formation of U(VI) peroxide compounds (J. A. Ridenour and C. L. Cahill, New Journal of Chemistry, 2018, 42, 1816-1831). For instance, previous reports in the literature have shown transformation of U(VI)-organic complexes to peroxides; however, this transformation is limited to the formation of small molecular units (dimers) (A. Blanes Diaz, D. V. Kravchuk, A. A. Peroutka, E. Cole, M. C. Basile and T. Z. Forbes, European Journal of Inorganic Chemistry, 2021, 2021, 166-176).

Irradiation is not a well-studied synthetic methodology due to the specialized facilities needed for these experiments. In this case, recreating the exact radiolytic conditions in the lab setting is more challenging and initial reports focused on direct irradiation of uranium-bearing media (A. Canizarès, et al., Journal of Raman Spectroscopy, 2012, 43, 1492-1497; and C. Jégou, et al., Journal of Nuclear Materials, 2005, 341, 62-82). However, a radical initiator method that mimics the lower levels of peroxide generated in an irradiation study has been developed (D. V. Kravchuk and T. Z. Forbes, Angewandte Chemie International Edition, 2019, 58, 18429-18433). While gamma irradiation of oxygenated solutions results in continuous peroxide generation at levels of 1.25·10⁻⁴ M (P. A. Yakabuskie, J. M. Joseph, C. R. Stuart and J. C. Wren, The Journal of Physical Chemistry A, 2011, 115, 4270-4278.), the autoxidation of benzaldehyde via a radical pathway generates comparable levels of peroxide in solution (˜2·10⁻⁵M) and results in the formation of U(VI) peroxide phases (D. V. Kravchuk and T. Z. Forbes, Angewandte Chemie International Edition, 2019, 58, 18429-18433). The major drawback to this methodology is the need for non-aqueous chemistry to sustain the radical initiator process.

Mechanochemistry uses mechanical energy (e.g., ball milling) to induce a chemical reaction. It is considered a green technology, because the solvent waste is minimized compared with traditional solution-based chemical reactions (J.-L. Do and T. Fris̆c̆ić, ACS Central Science, 2017, 3, 13-19). It has been successfully utilized in a range of materials (T. Fris̆c̆ić, et al., Angewandte Chemie International Edition, 2020, 59, 1018-1029), but is underexplored within the actinide chemistry. Reports on mechanochemistry of uranium focus on grinding simple uranium oxides or salts (e.g. UO₃, U₃O₈, and UO₂(NO₃)₂.6H₂O) (P. Kovacheva, et al., Journal of Radioanalytical and Nuclear Chemistry, 2007, 274, 481-490; P. Kovacheva, et al., Journal of Radioanalytical and Nuclear Chemistry, 2007, 274, 473-479; and P. Kovacheva, et al., Journal of Radioanalytical and Nuclear Chemistry, 2011, 287, 193-197), chelation of U(III) using borohydride ligands (T. V. Fetrow and S. R. Daly, Dalton Transactions, 2021, 50, 11472-11484; and T. V. Fetrow, et al., Inorganic Chemistry, 2020, 59, 48-61), complete/partial chlorination of U₃O₈ (S. Kitawaki, et al., Journal of Nuclear Materials, 2013, 439, 212-216), or synthesis of alternative U-based fuels (G. A. Alanko and D. P. Butt, Journal of Nuclear Materials, 2014, 451, 243-248).

Currently there is a need for improved methods that can be used to form actinide metal peroxides (e.g., uranium peroxides) without the use of liquids. Such methodology would decrease the need for the use of strong acids in the production or reprocessing of actinide metal oxides (e.g. uranium-based fuels for nuclear reactors).

SUMMARY

Using the methods of the invention, mechanochemistry has been used to generate actinide peroxides from U(IV), U(VI), and mixed valent (U(IV)/U(VI) oxides.

The invention provides the first mechanochemical synthesis of crystalline U(VI) peroxide phases by rotary milling of uranium trioxide UO₃ powders with solid metal peroxides Me_(x)O₂ (Me=Li, Na, Mg, Ca, Sr, Ba). Thus, the invention provides a mechanical method that can be used to form actinide peroxides (e.g., uranium triperoxide) without the use of liquids.

In one aspect the present invention provides a method for preparing an actinide metal peroxide, comprising contacting an actinide metal oxide with a solid metal peroxide under conditions that provide the actinide metal peroxide.

In another aspect, the invention provides an actinide metal peroxide prepared according to the method of the invention.

DETAILED DESCRIPTION

The following definitions are used, unless otherwise described.

The term “actinide metal” includes Uranium and Plutonium. In one embodiment, the actinide metal is Uranium.

The term “actinide metal oxide” includes UO₂, UO_(2+x), U₄O₉, U₃O₇, U₃O₈, UO₃, PuO₂, and PuO_(2+x). In one embodiment, the actinide metal oxide is actinide VI metal oxide. In one embodiment, the actinide metal oxide is actinide IV metal oxide. In one embodiment, the actinide metal oxide is UO₃. In one embodiment, the actinide metal oxide is UO₂.

The term “liquid-assisted grinding” (solvent-drop grinding) refers to an extension of solvent-free mechnanochemical methods, where a small amount of liquid is introduced to the milling reaction in order to enhance the reactivity of reagents. Parameter η refers to the ratio of the volume of liquid additive to the combined weight of the reactants η=V_(liquid)(μL)/m_(reagents) (mg). While η=0 μL/mg refers to neat solid on solid grinding, the typical values for liquid-assisted grinding are in the range η=0-2 μL/mg . In one embodiment, the method comprises liquid-assisted grinding.

The term “solid metal peroxide” includes any solid metal peroxide that is capable of converting an actinide metal oxide to an actinide triperoxide. In one embodiment, the solid metal peroxide has the formula M_(x)O₂, wherein M is Li, Na, Mg, Ca, Sr, or Ba. In one embodiment, M is Li or Na.

The term “rotary milling” includes the grinding of solid materials with or without a small amount of liquid in a mill to create smaller particles of the sample or start a mechanochemical reaction. In one embodiment, the rotary milling occurs in a ball mill or planetary ball mill. In one embodiment, the rotary milling occurs in a vertical roller mill.

In one embodiment, the method is carried out without solvent.

The invention will now be illustrated by the following non-limiting Examples.

EXAMPLES Example 1. Preparation Of Uranium Triperoxide from UO₃ Materials

Solid-state mechanochemical reactions were prepared using powders of uranium trioxide (International Bio-Analytical Industries Inc., 99%), lithium peroxide (Acros Organics, 95%), sodium peroxide (Acros Organics, 96%), magnesium peroxide complex (Aldrich, technical grade), calcium peroxide (Spectrum, 60%), strontium peroxide (Aldrich, 98%), barium peroxide (Acros Organics, 95%). Caution: UO₃ contains radioactive ²³⁸U, which is an a emitter, and like all radioactive materials must be handled with care. These experiments were conducted by trained personnel in a licensed research facility with special precautions taken toward the handling, monitoring, and disposal of radioactive materials. Recrystallization reactions were prepared using ultrapure Millipore water (18.2 MΩ). Chemicals purchased were used directly without further purification.

Mechanochemical Synthesis

Synthetic procedure for m₃₀-UO₃—Li₂O₂ involved combining 0.03 gr of UO₃ solid (1.10⁻⁴ moles) with 0.05 gr of Li₂O₂ solid (1.10⁻³ moles) in the 5 mL stainless steel FTS “SmartSnap” grinding jars using two 5 mm stainless steel balls (304 grade) for grinding. The grinding jars were sealed using a Teflon insert and inserted into a Form-Tech Scientific (FTS) FTS1000 shaker mill operating at 1800 rpm. The samples were ground in 5-minute grinding intervals for the total of 15 minutes or 30 minutes. Reactions between UO₃ and Na₂O₂, MgO₂, CaO₂, SrO₂, and BaO₂ were performed according to the same procedure with varying the total time of grinding to optimize the yield of the reactions, if applicable.

Vibrational Spectroscopy

Solid-state and solution-state Raman spectra were acquired on a SnRI High-Resolution Sierra 2.0 Raman spectrometer equipped with 785 nm laser energy and 2048 pixels TE-cooled CCD. Laser power was set to the maximum output value of 15 mW, giving the highest achievable spectral resolution of 2 cm⁻¹. Each sample was irradiated for an integration time of 5-60 seconds (depending on the sample) and automatically reiterated three times in multi-acquisition mode. Three Raman spectra acquired per sample, averaged together, and normalized based on laser power and integration time. FT-IR spectra were collected on a Nicolet Nexus FT-IR Spectrometer from 500-4000 cm⁻¹. Approximately 10-20 mg of the fine reaction powder was mixed with the anhydrous KBr salt and pressed into a transparent pellet for data collection. To accurately process the vibrational signals observed, the background was subtracted, multiple peaks were fit using the peak analysis protocol with Gaussian and Lorentz functions, and all the fitting parameters converged in the OriginPro 9.1.0 (OriginLab, Northampton, Mass.) 64-bit software.

Powder X-Ray Diffraction

Powder X-ray diffraction data was collected on a Bruker D8 Advance diffractometer with Ni-filtered Cu Kα radiation (λ=1.5418 Å), voltage 40 kV and current 40 mA, in the continuous mode with scan range of 5-60° 2θ and step size of 0.05°. Samples were ground to fine powders using a mortar and pestle and run using zero background silica sample holders. Processing of the PXRD patterns, background subtraction, smoothing, Kα2 stripping, and peak selection was done using PreDICT indexing software from ICDD. The PXRD diffractograms were matched using the ICDD PDF-4+ database.

Scanning Electron Microscopy

Secondary electron and backscattered electron images were obtained with a Hitachi 3400N Scanning Electron Microscope for each sample. Reaction powders were removed from the grinding jar and directly adhered to a SEM stub by double-sided carbon tape. The operating voltage and the emission current were 15.0 kV and 80-120 μA respectively.

Results and Discussion

The mechanochemical reaction between UO₃ and Li₂O₂ powders resulted in a crystalline product after 30 minutes of rotary grinding at 1800 rpm (m₃₀-UO₃—Li₂O₂). The powder X-ray diffraction pattern of the solid product matched to the known crystalline phase [UO₂(O₂)₃]₁₂[(UO₂(OH)₄)Li₁₆(H₂O)₂₈]₃.Li₆[H₂O]₂₆, otherwise known as ULi₁₆ with no evidence of the U(VI) starting material (M. Nyman, et al., Inorganic Chemistry, 2010, 49, 7748-7755). ULi₁₆ is a nanoclustered assembly of uranyl triperoxide UO₂(O₂)₃ ⁴⁻ and uranyl tetrahydroxide UO₂(OH)₄ ²⁻ anionic monomers counterbalanced with extensive hydrated lithium frameworks. Vibrational spectra were complicated by the bands at 814 cm⁻¹ and 864 cm⁻¹ in the IR spectrum and 789 cm⁻¹ in the Raman spectrum due to the presence of unreacted Li₂O₂ (H. H. Eysel and S. Thym, Zeitschrift für anorganische and allgemeine Chemie, 1975, 411, 97-102). Raman spectrum of ULi₁₆ product showed two bands at 715 and 737 cm⁻¹ that corresponded to the ν₁(U═O) stretch of the UO₂(O₂)₃ ⁴⁻ species (M. Dembowski, et al., Inorganic Chemistry, 2017, 56, 1574-1580). Low vibrational frequency of the uranyl symmetric stretch is rather common among uranyl triperoxide complexes due to significant σ-electron donation by the peroxide O₂ ²⁻ ligands into the equatorial plane of the UO₂ ²⁺ cation (G. Lu, et al., Coordination Chemistry Reviews, 2018, 374, 314-344). Such additional σ-donation causes weakening of the uranyl (U═O_(y1)) bond and enables the oxo group to engage in actinyl-cation interactions with surrounding Li⁺ cations. The band at 767 cm⁻¹ could be assigned to residual, amorphous UO₃; however, due to the large FWHM of the peak, it could also be associated with the ν₁(U═O) stretch of a UO₂(OH)₄ ²⁻ phase in the final product. Finally, Raman band at 844 cm⁻¹ is well-aligned with symmetric ν₁(O—O) of the peroxide ligand bound to the uranyl cation (M. Dembowski, et al., Inorganic Chemistry, 2017, 56, 1574-1580). Weak band at 1090 cm⁻¹ corresponded to the CO₃ ²⁻ stretching of Li₂CO₃ that is produced as a byproduct of reaction between activated Li₂O₂ and molecular CO₂ from open air. The IR spectrum for mechanochemically synthesized m₃₀-UO₃—Li₂O₂ was well-matched to the vibrational bands of ULi₁₆ synthesized in solution from previous literature reports (M. Nyman, et al., Inorganic Chemistry, 2010, 49, 7748-7755).

While the mechanochemical reaction between UO₃ and Li₂O₂ took 30 minutes to create a highly crystalline product, a similar product was noted only after 15 minutes of rotary grinding of UO₃ with Na₂O₂ (m₁₅-UO₃—Na₂O₂). The PXRD pattern of the crystalline product matched the Na₄[UO₂(O₂)₃].9H₂O phase (NaUT), reported by Dembowski et al. (M. Dembowski, et al., Inorganic Chemistry, 2017, 56, 1574-1580). The synthesized m₁₅-UO₃—Na₂O₂ phase once again contains uranyl triperoxide monomers UO₂(O₂)₃ ⁴⁻ that are charge balanced by Na⁺ cations and engage in hydrogen bonding interaction with water molecules within the crystalline lattice. In case of m₁₅-UO₃—Na₂O₂, the analysis of vibrational spectra was more straightforward because bands associated with the Na₂O₂ starting material was not observed in this case. The major feature in the Raman spectrum of m₁₅-UO₃—Na₂O₂ was centred at 701 cm⁻¹ and corresponded to the ν₁(U═O) stretch of the uranyl cation. The vibrations at 808, 819, and 842 cm⁻¹ corresponded to ν₂, ν₃, and ν₁(O—O) stretches of the coordinated peroxide ligands O₂ ²⁻ based on previously reported computational results (M. Dembowski, et al., Inorganic Chemistry, 2017, 56, 1574-1580). A weak band at 770 cm⁻¹ was assigned to a minor amount of unreacted UO₃ and a medium intensity band at 1080 cm⁻¹ corresponded to the carbonate CO₃ ²⁻ in form of sodium carbonate Na₂CO₃. A notable feature in the IR spectrum of m₁₅-UO₃—Na₂O₂ is the red-shifted asymmetric ν₃(U═O) stretch at 865 cm⁻¹, following a trend of the red-shifted symmetric ν₁(U═O) stretches in the Raman spectra.

Increasing the reaction time of the UO₃—Na2O₂ solid mixture to 30 minutes (m₃₀-UO₃—Na₂O₂) resulted in the formation of a different crystalline product mixture as evidenced by the experimental PXRD pattern. The powder pattern of m₃₀-UO₃—Na₂O₂ exhibits preferred orientation of particles and addition peaks in the diffractogram also indicate that the resulting phase was not a monophasic compound (C. F. Holder and R. E. Schaak, ACS Nano, 2019, 13, 7359-7365). Raman spectroscopy indicated that a uranyl triperoxide species (UO₂(O₂)₃ ⁴⁻) was present in the solid phase based on similar spectroscopic features at 701 and 722 cm⁻¹ associated with the symmetric ν₁(U═O) stretch of the uranyl cation coordinated to three O₂ ²⁻ ligands. Vibrational bands associated with peroxide ligands were assigned to 790 (ν₂), 808 (ν₃), and 838 cm⁻¹ (ν₁), whereas the peak at 767 cm⁻¹ can again be assigned with residual UO₃. The most unusual feature within the Raman spectrum of m₃₀-UO₃—Na₂O₂ was a weak band at 878 cm⁻¹ that has been previously observed for uranyl superoxide complex (D. V. Kravchuk, et al., Angewandte Chemie International Edition, 2021, 60, 15041-15048). Mechanochemically induced radical formation is particularly common for molecules containing peroxides or disulfides (G. Kaupp, CrystEngComm, 2009, 11, 388-403); thus, formation of reactive oxygen species from metal peroxide powders upon mechanical grinding is plausible. Presence of superoxide radicals within the m₃₀-UO₃—Na₂O₂ powder may explain the high reactivity of the mechanochemical reaction. In addition, ingrowth of carbonate in the solid-state compound was observed based on Raman bands at 1080 and 1057 cm⁻¹ that correspond to the ν₁ stretch of CO₃ ²⁻ associated with Na⁺ and UO₂ ²⁺, respectively. Capture of molecular CO₂ from air by the powder mixture is most likely a due to the creation of the reactive oxygen species and NaOH, which can form when Na₂O₂ is exposed to moisture in air.

Additionally, the mechanochemical reactions between UO₃ and solid alkali-earth metal peroxides (MgO₂, CaO₂, SrO₂, BaO₂) were explored under similar synthetic conditions. PXRD patterns and Raman analysis suggests that reactivity of alkali-earth metal peroxides increases down the group with MgO₂ being least reactive and BaO₂ being most reactive. In case of MgO₂ the resulting reaction product is still a mixture of starting materials. Moving down the group, PXRD patterns of product powders show decrease in crystallinity, as well as systematic decrease in vibrational signatures of UO₃ starting material (˜6 vibrational bands in the 200-600 cm⁻¹ region). The reaction between UO₃ and BaO₂ results in UO₃ powder losing crystallinity and becoming completely amorphous as the diffraction peaks associated with uranium trioxide completely disappear from the diffractogram. Despite the increase in reactivity down the group, mechanochemical reactions with alkali-earth metal peroxide solids do not yield any U(VI) peroxide materials under the conditions explored herein.

The formation of the U(VI) triperoxide phase in the case of m₃₀-UO₃—Li₂O₂ and m₁₅-UO₃—Na₂O₂ has important implications to the development of starting material for use in the synthesis of larger U(VI) peroxide nanospheres or other phases. Monomeric U(VI) triperoxides have previously been shown to exhibit aqueous reactivity and self-assemble into peroxide/hydroxide bridged uranyl nanosphere polyoxometalate clusters over time (A. Arteaga, et al., Chemistry—A European Journal, 2019, 25, 6087-6091; and M. Nyman, et al., Inorganic Chemistry, 2010, 49, 7748-7755). For instance, Liao et al. dissolved lithium U(VI) triperoxide solids [Li₄(UO₂)(O₂)₃.4H₂O] in water and a catalyst solution to study the self-assembly of the Li—U₂₄ nanocapsule. The triperoxide starting material in this case was produced by dissolving uranyl nitrate hexahydrate in a solution containing 3 mL 30% H₂O₂ and 3 mL of 4 M LiOH solution and resulted in product yields of 75% based upon U.

To evaluate the transformation of the mechanochemically produce triperoxide phases, the synthesized powders for m₃₀-UO₃—Li₂O₂ and m₃₀-UO₃—Na₂O₂ were each dissolved in 2 ml of water and the capped vial was allowed to sit undisturbed on the benchtop. Overnight both solutions changed color from light yellow to deep orange and Raman spectroscopy of the resulting solution showed a significant amount of free carbonate based on the ν₁ CO₃ ²⁻ band at 1068 cm⁻¹ (for m₃₀-UO₃—Li₂O₂ and m₃₀-UO₃—Na₂O₂ solutions, respectively). Ingrowth of the dissolved carbonate was somewhat surprising due to relative stability of uranyl triperoxide monomers in solutions up to 96 hours after the dissolution (Z. Liao, et al., Inorganic Chemistry, 2014, 53, 10506-10513). However, the presence of mechanochemically induced reactive oxygen species within the solid-state could cause formation of uranyl superoxide species, which in turn has been shown to capture carbon both in solid-state and in solution (D. V. Kravchuk, et al., Angewandte Chemie International Edition, 2021, 60, 15041-15048; and D. V. Kravchuk and T. Z. Forbes, ACS Materials Au, 2021, DOI: 10.1021/acsmaterialsau.1c00033).

Recrystallized powder of m₃₀-UO₃—Li₂O₂ appeared to be crystalline mixture of Li₂CO₃ and an unknown phase that could not be matched to any known compound in the in the ICDD PDF-4+ database. Raman spectrum of recrystallized m₃₀-UO₃—Li₂O₂ displayed vastly different vibrational features compared to the original powder. The major intensity band centred at 1091 cm⁻¹ (vi CO₃ ²⁻ bound to Li⁺) further confirmed a significant amount of Li₂CO₃ present in the mixture and the peak at 1063 cm⁻¹ corresponds to the carbonate ligand symmetric stretch of the uranyl tricarbonate species UO₂(CO₃)₃ ⁴⁻. This was well aligned with vibrational bands at 845 (ν₂ out-of-plane bend of CO₃ ²⁻), 744, and 711 cm⁻¹ (ν₄ in-plane bend of CO₃ ²⁻) (J. Cĕjka, et al., Journal of Raman Spectroscopy, 2010, 41, 459-464). The position of the uranyl symmetric stretch band at 816 cm⁻¹ in the Raman and antisymmetric stretch at 908 cm⁻¹ in the IR further suggested the formation of a U(VI) tricarbonate species (UO₂(CO₃)₃ ⁴⁻). As a result, the unidentified crystalline phase in the PXRD of recrystallized m₃₀-UO₃—Li₂O₂ likely has a general chemical formula of Li₄[UO₂(CO₃)₃].xH₂O; however, a mineral or synthetic phase of lithium uranyl tricarbonate has not been reported in the literature to confirm this assignment.

Powder X-ray diffraction of recrystallized m₃₀-UO₃—Na₂O₂ resulted in highly crystalline material that matched a known sodium U(VI) tricarbonate phase, Na₄[UO₂(CO₃)₃] (cĕjkaite). Vibrational signatures of carbonate anion were present in the Raman spectrum, including the symmetric stretching ν₁ at 1073 (bound to Na⁺) and 1063 cm⁻¹(bound to UO₂ ²⁺), as well as doubly-degenerate in-plane bending ν₄ at 702 and 734 cm⁻¹. Uranyl ν₁ symmetric stretch was centered at 806 cm⁻¹ in the Raman spectrum with a complementary ν₃ antisymmetric stretch showing up as a shoulder at 887 cm⁻¹ in the IR, both being characteristic of uranyl tricarbonate mineral phases (G. Lu, et al., Coordination Chemistry Reviews, 2018, 374, 314-344).

In both cases, the formation of larger U(VI) peroxide nanoclusters due to the presence of additional reactive oxygen species in the system was not observed. It is likely that if the experiment was performed in a CO₂ free atmosphere, then peroxide or novel materials containing reactive oxygen species may have been isolated from the solution. This methodology does lead to the formation of pure phase U(VI) tricarbonate species, which are somewhat difficult to form from aqueous solutions.

CONCLUSIONS

Mechanochemical synthesis has been successfully applied to UO₃ powders mixed with solid alkali metal peroxides, Li₂O₂ and Na₂O₂, resulting in crystalline uranyl triperoxide compounds [UO₂(O₂)₃]₁₂[(UO₂(OH)₄)Li₁₆(H₂O)₂₈]₃.Li₆[H₂O]₂₆ and Na₄[UO₂(O₂)₃].9H₂O, respectively. This is the first successful report using mechanochemistry to convert a U(VI) oxide phase to a crystalline peroxide product. The timeframe of the mechanochemical grinding played a significant role in driving the reaction to completion and controlling the overall product. In both cases, mechanicalochemical reactions of the solid metal peroxides likely induced the formation of reactive oxygen species, such as superoxide radical, which combined with alkalinity of reaction powders, resulting in carbon capture and formation of the carbonate anions. Both m₃₀-UO₃—Li₂O₂ and m₃₀-UO₃—Na₂O₂ reaction products were recrystallized in water yielding uranyl tricarbonate UO₂(CO₃)₃ ⁻ phases charge balanced by either Li⁺ or Na⁺ cations, as confirmed by powder X-ray diffraction and vibrational spectroscopy. Such mechanochemical synthetic pathway to uranyl triperoxides may not only afford access to new crystalline compounds, but also avoid significant generation of radioactive liquid waste in the laboratory setting.

Example 2. Preparation Of Uranium Peroxide from UO₂

Mechanochemical reactions were carried out on a mixture of 0.03 g of UO₂ with 0.05 g of Li₂O₂ (or a molar equivalent of Na₂O₂) for neat grinding and with addition of 100 μL of 30% H₂O_(2 (aq)) as a liquid additive for liquid-assisted grinding LAG, where parameter η≈1 μL/mg (P. Ying, J. Yu and W. Su, Advanced Synthesis & Catalysis, 2021, 363, 1246-1271). Liquid-assisted grinding (solvent-drop grinding) is an extension of solvent-free mechnanochemical methods, where a small amount of liquid is introduced to the milling reaction in order to enhance the reactivity of reagents. Parameter η refers to the ratio of the volume of liquid additive to the combined weight of the reactants η=V_(liquid)(μL)/m_(reagents) (mg). While η=0 μL/mg refers to neat solid on solid grinding, the typical values for liquid-assisted grinding are in the range η=0-2 μL/mg compared to η≈1 μL/mg, which is used in the experiments herein. Samples were ground for 30 minutes in the 5 ml stainless steel milling jar with four 5 mm stainless steel balls on the rotary mill at 1800 rpm.

Mechanochemical activation of UO₂ powder itself was first evaluated using powder X-ray diffraction with an addition of corundum Al₂O₃ standard (25% by weight) to assess changes in crystallinity across the samples. The initial PXRD pattern of UO₂ solid showed strong diffraction peaks associated with UO₂ confirming a highly crystalline starting material with weak and broadened peaks matching to UO₃ suggesting minor surface oxidation of UO₂ upon exposure to air during handling. The intensity ratio between the diffraction peak associated with (111) crystallographic plane of UO₂ (2θ=28.5°) and (104) plane of Al₂O₃ (2θ=35.1°) in the initial PXRD was calculated to be 24.6:1 (UO₂:Al₂O₃) (Y. Hu, X. Han, F. Cheng, Q. Zhao, Z. Hu and J. Chen, Nanoscale, 2014, 6, 177-180). The powder pattern of solid uranium dioxide ground for 30 minutes (m₃₀-UO₂) revealed a significant decrease in peak intensities, as well as, peak broadening, suggesting the decrease in crystallinity and possible nanoscale domains of coherent diffraction. The peak intensity ratio against the standard in case of m₃₀-UO₂ was decreased to 4.2:1 (UO₂:Al₂O₃), suggesting a decrease of crystallinity from the original sample by a factor of 5.8. The mechanochemical impact on solid UO₂ alone demonstrated breakdown of crystallinity, decrease of particle size, which leads to increased surface area and enhanced reactivity of ground UO₂ powders.

This was further confirmed after the liquid-assisted grinding reaction with Li₂O₂ (m₃₀-UO₂-Li₂O₂ LAG) resulting in a new uranium crystalline phase and complete disappearance of peaks associated with crystalline UO₂ in the powder diffraction pattern. Reaction with solid Li₂O₂ was first performed as neat grinding to assess the oxidative capabilities of Li₂O₂. Powder X-ray diffraction pattern of the resulting solid m₃₀-UO₂—Li₂O₂ showed diffraction peaks associated with starting materials UO₂ and Li₂O₂ indicating no formation of crystalline product. However, Raman spectroscopy on the solid product revealed partial oxidation of UO₂ in form of amorphous uranium oxides. Vibrational band at 443 cm⁻¹ was assigned to the stoichiometric UO₂, while the shoulder at 458 cm⁻¹ corresponded to non-stoichiometric UO_(2+x) (J. M. Elorrieta, L. J. Bonales, M. Naji, D. Manara, V. G. Baonza and J. Cobos, Journal of Raman Spectroscopy, 2018, 49, 878-884). The band at 627 cm⁻¹ matched very well with U₄O₉ formed during oxidation of UO₂ (J. M. Elorrieta, L. J. Bonales, M. Naji, D. Manara, V. G. Baonza and J. Cobos, Journal of Raman Spectroscopy, 2018, 49, 878-884). Bands at 562 cm⁻¹ and 593 cm⁻¹ can possibly correspond to vibrational bands of U₃O₇ and U₃O₈ (J. B. Lü, G. Li and S. L. Guo, Guang Pu Xue Yu Guang Pu Fen Xi, 2014, 34, 405-409). Weak bands at 717 cm⁻¹ and 730 cm⁻¹ match well with vi symmetric stretch of uranyl U═O bound to three peroxide ligands, suggesting minimal conversion to U(VI) triperoxide phase. A strong band at 785 cm⁻¹ is associated with the ν₁(O—O) of peroxide anion in the Li₂O₂ starting material. Band at 1090 cm⁻¹ corresponds to ν₁ stretching mode of CO₃ ²⁻ bound to lithium. Mechanochemical impact on solid peroxide results in the formation of reactive oxygen species such as superoxide anions, that readily react with molecular CO₂ resulting in the ingrowth of carbonate phases (G. Kaupp, CrystEngComm, 2009, 11, 388-403; D. V. Kravchuk and T. Z. Forbes, ACS Materials Au, 2021, DOI: 10.102 1/acsmaterialsau.1c00033). As a result, the neat grinding in case of m₃₀-UO₂—Li₂O₂ does not produce any crystalline product, however, vibrational spectroscopy suggests evidence of partially oxidized UO₂.

Liquid-assisted grinding approach with Li₂O₂ demonstrated significant ingrowth of crystalline material in the PXRD pattern of m₃₀-UO₂—Li₂O₂ LAG, while the peaks associated with crystalline UO₂ disappeared. The PXRD pattern of m₃₀-UO₂—Li₂O₂ LAG was not matched to any previously reported materials in the ICDD PDF-4+ database. However, Raman spectroscopy of the solid m₃₀-UO₂—Li₂O₂ LAG revealed two major vibrational bands at 716 cm⁻¹ and 735 cm⁻¹ aligning with ν₁ symmetric stretch of U═O for uranyl triperoxide species (M. Dembowski, V. Bernales, J. Qiu, S. Hickam, G. Gaspar, L. Gagliardi and P. C. Burns, Inorganic Chemistry, 2017, 56, 1574-1580). The band at 786 cm⁻¹ was still associated with ν₁ stretching of the peroxide anion from the Li₂O₂ starting material, however, the band at 847 cm⁻¹ corresponded to the symmetric O—O stretching of peroxide ligand bound to the uranyl cation, thus confirming the presence of uranyl triperoxide species. Liquid-assisted grinding was demonstrated to enhance the reactivity of UO₂ with Li₂O₂ under the same reaction conditions resulting in the formation of U(VI)O₂(O₂)₃ ⁴⁻ species.

Oxidation of UO₂ was reasonably enhanced during neat grinding with Na₂O₂ as compared to the Li₂O₂ counterpart. The PXRD pattern of the product m₃₀-UO₂—Na₂O₂ still showed major diffraction peaks associated with UO₂ starting material, however, additional weaker peaks suggested ingrowth of a new crystalline phase that partially matched to Na₄[UO₂(O₂)₃].9H₂O uranyl triperoxide phase reported previously (N. W. Alcock, Journal of the Chemical Society A: Inorganic, Physical, Theoretical, 1968, DOI: 10.1039/J19680001588, 1588-1594). Raman spectroscopy of m₃₀-UO₂—Na₂O₂ revealed a complex mixture of uranium oxides, similar to the one observed in m₃₀-UO₂—Li₂O₂. Vibrational bands at 443 cm⁻¹ and 458 cm⁻¹ were assigned to UO₂ and UO_(2+x), respectively, while a band at 627 cm⁻¹ was aligned with U₄O₉. A prominent band at 698 cm⁻¹ was associated with the uranyl symmetric stretch, while weaker bands at 810 cm⁻¹ and 838 cm⁻¹ corresponded to ν₃ and ν₁ O—O stretching of peroxide ligands, indicating a presence of uranyl triperoxide species. The neatly ground m₃₀-UO₂—Na₂O₂ displayed vibrational bands associated with the symmetric stretch of superoxide anion O₂ ⁻· centered at 1136 cm⁻¹ and 1152 cm⁻¹(H. H. Eysel and S. Thym, Zeitschrift für anorganische and allgemeine Chemie, 1975, 411, 97-102). Presence of reactive oxygen species, such as superoxide anion, in the solid, suggests a mechanochemically induced electron transfer from the peroxide in the starting material to UO₂ solid, resulting in U(V) and U(VI) phases. The remaining superoxide radical can potentially undergo another electron transfer reaction with U(IV) or U(V) and leave the solid as molecular oxygen gas. A different reaction pathway for superoxide is reactivity with CO₂ from air resulting in the formation of CO₃ ²⁻, presence of which is also evident in vibrational spectroscopy of m₃₀-UO₂—Na₂O₂ by the strong band at 1080 cm⁻¹.

Utilizing the liquid-assisted grinding approach with Na₂O₂ has further improved the crystallinity of the new uranium phase seen in the PXRD pattern of m₃₀-UO₂—Na₂O₂ LAG. The major diffraction peaks fully matched the sodium uranyl triperoxide phase Na₄[UO₂(O₂)₃].9H₂O confirming that the crystalline phase is fully converted from U(IV) to U(VI). Raman spectroscopy has supported the uranyl triperoxide species based on the vibrational bands centered at 695 cm⁻¹ (vi symmetric U═O stretch), 813 cm⁻¹(ν₃ O₂ ²⁻ uranyl), and 842 cm⁻¹ (ν₁ O₂ ²⁻ uranyl). The band at 877 cm⁻¹ is most likely associated with free peroxide anion due to hygroscopic surface of the solid sample, however, we previously observed a band associated with uranyl superoxide stretch at 880 cm⁻¹, so the precise assignment of the band may be ambiguous (D. V. Kravchuk, N. N. Dahlen, S. J. Kruse, C. D. Malliakas, P. M. Shand and T. Z. Forbes, Angewandte Chemie International Edition, 2021, 60, 15041-15048).

All publications, patents, and patent documents (including Kravchuk, D. and Forbes, T. Chem. Comm., 2022, 58, 4528-4531 and Kravchuk, D. and Forbes, T., CrystEngComm, 2022, 24, 775-781) are incorporated by reference herein, as though individually incorporated by reference. The invention has been described with reference to various specific and preferred embodiments and techniques. However, it should be understood that many variations and modifications may be made while remaining within the spirit and scope of the invention. 

What is claimed is:
 1. A method for preparing an actinide metal peroxide, comprising contacting an actinide metal oxide with a solid metal peroxide under conditions that provide the actinide metal peroxide.
 2. The method of claim 1, wherein the actinide metal is uranium.
 3. The method of claim 1, wherein the actinide metal oxide is an actinide VI metal oxide.
 4. The method of claim 1, wherein the actinide metal oxide is an actinide IV metal oxide.
 5. The method of claim 1, wherein the solid metal peroxide comprises Li, Na, Mg, Ca, Sr, or Ba.
 6. The method of claim 5, wherein the solid metal peroxide comprises Li₂O₂ or Na₂O₂.
 7. The method of claim 1, wherein the actinide metal oxide and the solid metal peroxide are contacted by rotary milling.
 8. The method of claim 1, further comprising converting the actinide metal peroxide to a solid.
 9. The method of claim 8, wherein the solid is a crystalline or amorphous solid.
 10. The method of claim 1, which is carried out without solvent.
 11. The method of claim 1, which comprises liquid-assisted grinding
 12. An actinide metal peroxide prepared according to the method of claim
 1. 